1. Field of the Invention
The present invention relates to a multiple-gap electric rotating machine advantageously used as an electric motor or an alternator for a vehicle.
2. Description of Related Art
Japanese Patent Application Laid-open No. 2007-282331 describes a double stator type motor as such a multiple-gap motor. This motor includes an annular rotor constituted of a laminated core of core sheets made of electromagnetic steel, permanent magnets fitted to the inner and outer peripheries of the laminated core, an inner stator disposed radially inside the rotor with a gap therebetween and an outer rotor disposed radially outside the rotor with a gap therebetween. This motor achieves high output power density by the provision of the two gaps between the rotor and the two stators.
The rotor of the motor described in the above patent document has the structure in which two core-holding plates made of aluminum are disposed on both sides of the laminated core, and a coupling disc disposed outside one of the core-holding plates. The laminated core is fastened and fixed to the coupling disc using stud bolts. That is, the rotor has the cantilever structure where it is fastened to the coupling disc at one axial end thereof at which one of the core-holding plates is disposed, and is free at the other axial end thereof at which the other core-holding plate is disposed. In the rotor having such a cantilever structure, the laminated core tends to extend radially outside at the open end thereof due to centrifugal force when the rotor rotates. Although the rotor described in the above patent document includes the aluminum-made core-holding plate disposed on the open end of the laminated core, since the Young's modulus of aluminum is small, it is not possible to sufficiently suppress the extension of the laminated core at the open end thereof.
Further, since the core-holding plates disposed on both sides of the laminated core are made of aluminum, the axial fastening force (pressing force) of the stud bolts is insufficient, and accordingly the holding friction force between the core sheets is weak. Therefore, the rotor described in the above patent document has a problem in that the rigidity of the rotor decreases when the motor rotates and the laminated core cannot endure the centrifugal force applied to the permanent magnets of the rotor. The above patent document describes, as an embodiment, a wheel-in motor which is incorporated in a drive wheel to drive the drive wheel. In this embodiment, the usable speed range of the wheel-in motor is about from 1000 to 2000 rpm. The rotor structure described in the above patent document is applicable to a low speed motor. However, it cannot be applied to a motor directly coupled to an engine whose rotational speed rises up to 7000 rpm.